Apparatus for the ignition of direct current welding and cutting arcs

ABSTRACT

A system for igniting direct current arcs for welding or cutting of metals, especially plasma welding, cutting and metal treatment and T.I.G. welding, wherein an elevated DC potential of 150 to 2,000 volts is temporarily superposed upon the welding potential of up to 100 volts during the ignition stage and is terminated upon detection of current flow between the electrode and the workpiece via an electronic switch adapted to cut out a primary winding or cut in a secondary winding of a transformer interposed between the line current source and the rectifier. The system is used primarily for the closed chamber welding of metallic cases or shells of fuel elements for a nuclear reactor core under a blanket of helium.

United States Patent [151 3,641,311

Hildebrandt et a1. 45 F b, 8, 1 97 2 [5 APPARATUS FOR THE IGNITION OF Rrenc s Cited 2335;552:22 WELDING AND UNITED STATES PATENTS 3,263,1577/1966 Klein ..323/22 [72] Inventors: Peter Hildebrandt, lsmaning;Walter 3,356,928 12/ 1967 Parrish ..219/1 31 X Ruckdeschel, Munich, bothof Germany Primary Examiner-J. V. Truhe [73] Asslgme: LmdeAkueugeseuschafi, Hollnegel' Assistant Examiner-C. L. Albritton skl'euthGermany Attorney-Karl F. Ross 'l d: [22] F1 e July 9, 1968 ABSTRACT [211 App! 743347 A system for igniting direct current arcs for welding orcutting of metals, especially plasma welding, cutting and metal treat-[30] Foreign Application Priority D t ment and T.I.G. welding, whereinan elevated DC potential of 150 to 2,000 volts is temporarily superposedupon the welding July 1 l, 1967 Germany ..P l6 15 364.3 potential f up t100 olt during the ignition stage and i 'minated upon detection ofcurrent flow between the electrode Cl r t t v /131 and the workpiece viaan electronic switch adapted to cut out [51] Int. Cl 1 ..B23k 9/10 aprimary winding or cut in a secondary winding of a trans- [58] Field ofSearch ..219/ 130, 131, 135, 137; former interposed between the linecurrent source and the 323/22, 435, 44 rectifier. The system is usedprimarily for the closed chamber welding of metallic cases or shells offuel elements for a nuclear reactor core under a blanket of helium.

4 Claims, 4 Drawing Figures PATENTEDFEB 8 m2 SHEET 1 BF 2 FIG.

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PETER HILDEBRANDT WALTER RUCKDESCHEL INVENTOR 5.

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ATTORNEY APPARATUS FOR THE IGNITION OF DIRECT CURRENT WELDING ANDCUTTING ARCS Our present invention relates to a method of and a circuitfor the ignition of direct current are for the DC arc welding or arecutting of metallic workpieces and especially for the plasma welding,cutting or surface treatment of a metal and T.l.G. welding (i.e.,tungsten inert gas welding) wherein the electrode sustaining the arcdoes not contact the workpiece but is maintained at a distance therefromwhile the arc spans the gap between electrode and workpiece.

Numerous direct current arc welding and cutting systems, including thosementioned above, have been commonly used heretofore and arecharacterized by the maintenance of an arc gap filled with a gas whichis ionized so as to render it conductive and permit the generation of anarc or another conductive path between an electrode (generally of thenonconsumable type but often also a consumable electrode rod). In someof these known systems, a high-frequency ignition circuit is providedwhich is designed to apply a high-frequency potential across theelectrode and the workpiece adapted to generate a bundle of sparks andthereby ionize the gas between electrode and workpiece to permitcontinued passage of the arc. In another earlier method, generally usedin plasma welding with throughgoing arcs, an auxiliary discharge isproduced by high frequency to ignite the main are between electrode andworkpiece with resulting ionization of the gas therebetween.

We have found that these techniques are not always successful increating a disturbance-free initial ignition of the arc. Such systemsare least effective in so-called chamber" welding which has come to thefore in recent years. In chamber welding, a protec,ive gas blanket,generally of an inert gas, is introduced into a closed chambercontaining the workpiece and prevents interaction between the weldmentand heated portions of the workpiece and atmospheric air which usuallycontacted the weldment and the workpiece in earlier welding systems. Thegas blanket has been found to affect detrimentally earlier efforts atignition of the DC arc.

It is, therefore, the principal object of our invention to provide animproved method of and system for the ignition of direct currentelectric arcs.

A more specific object of this invention is to provide a system for theignition of direct current arcs, especially DC welding and cutting arcsfor T.I.G. and plasma welding, which allows a disturbance-freegeneration of the initial ionization state or arc between electrode andworkpiece.

Yet another object of our invention is to provide an ignition system forthe purposes described which will be efi'ective to produce the desiredare without differences in the ignition characteristics resulting fromthe use of protective gas blankets of different types and compositions.

A further object of the instant invention is to provide an improvedmethod of welding the metal cans, shells or cases of fuel elements fornuclear reactor cores without endangering operating personnel.

These objects and others, which will become apparent hereinafter, areattainable in accordance with the present invention which provides thatan elevated direct current voltage, derived from a source other than thesource of the operating direct current voltage, is applied across theworkpiece and the arc electrode for a brief period at the inception ofwelding or cutting and in the absence of a discharge to effecthigh-voltage breakdown of the gas in the gap between the electrode andthe workpiece, this auxiliary high-voltage source being disconnected assoon as the ionization state in the gap allows conduction thereacrossand the consequent establishment of the arc. More specifically, we havefound that, with the usual operating characteristics of welding DC arcsusing potentials up to 100 volts, it is possible to obtaindisturbance-free ignition independently of the gas conditions in thegap, by applying thereacross an elevated DC voltage of 150 to 2,000volts, preferably 200 to 800 volts.

The present system has been found to be especially advantageous for theignition of DC arcs between a welding or cutting electrode and aworkpiece spaced therefrom in protective gas atmospheres of an inertgas, especially helium. According to a more specific feature of thisinvention, therefore, the welding process is carried out with a T.I.G.(tungsten inert gas) or plasma-welding head in a sealed chamber to whichhelium is supplied as the protective gas. The system has been found tobe extremely important for the welding of fuel element shells fornuclear reactors since continuity of welding and the quality of the weldare such that such elements can be sealed with assurance and minimumdisturbance from the protective gas. In addition, this techniqueeliminates all danger to operating personnel who remain outside theclosed chamber and manipulate the welding head and the workpiece fromthe exterior.

When the system of the present invention is carried out in the open,however, it is important to prevent contact of the welding head or theworkpiece by operating personnel because of the high voltages appliedduring the inception of ionization. No such problem arises in closedchamber welding as previously mentioned. The termination of thesuperimposed high-voltage direct current is, according to the presentinvention, effected within an interval of at most one second after ithas been applied and, as a rule, within an interval of tenths of asecond. In some cases, it is possible to reduce this interval to theorder of milliseconds.

The ignition circuit, according to the present invention, includes awelding current source having a sharply decreasing voltagecharacteristic and can include a transformer having a primary windingconnected to the alternating current line source and a secondary windingconnected across a rectifier bridge whose output terminals are appliedacross the electrode and the workpiece; the transformer can be of thefield dissipative type to ensure the steep characteristic. The desiredsharply decreasing voltage characteristic can be realized also byproviding in series with one of the windings of the transformer anappropriately dimensioned choke or ohmic resistance.

According to another feature of this invention, the transformer has aprimary winding and a secondary winding at least one of which is tappedand provided with an electronic switch to cut in or cut out one of thetransformer sections, thereby decreasing the ratio of effectivesecondary turns to primary turns in response to the detection of acurrent flow in the output circuit of the rectifier. An electronicswitch consisting of a pair of bucking solid-state controlled rectifiersor thyristors, may be used for this purpose while a relay of theelectromagnetic type may be employed in conjunction with the electronicswitch or independently thereof. The output circuit of the system thuscan include a choke connected in series with one output terminal of therectifier bridge and to the workpiece while the other output terminal ofthe rectifier bridge is connected to the electrode. In this seriescircuit, there is further included a current responsive detector foroperating the electronic switch. The detector may be connected to thethyristor switch via a time constant network with an externally smalltime constant (e.g., from the order of milliseconds to the order oftenths of a second), so that the effective duration of the elevatedpotential will be of the magnitude indicated previously. Consequently,the switching circuit has a steep characteristic of the desired slope.It will be understood also that a completely independent source may beprovided for the arc ignition voltage.

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a circuit diagram illustrating the problems of the presentinvention and in which the electronic switch is provided in the primarycircuit;

FIG. 2 is another circuit embodying the principles of the presentinvention and having the electronic switch connected in the secondarycircuit;

FIG. 3 represents still another arrangement for igniting DC arcs; and

FIG. 4 shows a circuit having distinct sources for the ignitingpotential and the welding potential.

In FIG. 1, we have shown a circuit in which the welding transformer 101has a tapped primary winding 111 whose sections are represented at 111aand HM while the output cited in the system is shown at 105 and includesthe secondary winding 112 of this transformer. The primary circuit 104includes a source of alternating line current 104a connected acrosssection 111a of primary winding 111 in series with a relay switch 103.

The secondary circuit 105 includes a rectifier bridge 106 whose inputterminals are tied across the secondary winding 112 of transformer 101and whose output terminals 106 are connected to an electrode 113 of aplasma or T.I.G. welding head spaced across a gap 1 15 from theworkpiece 1 14.

The workpiece is connected in series with a current responsive detector109 (e.g., a current transformer ora lw-ohmic resistor as shown in FIG.3) and a choke 108 to the other output terminal of the rectifier bridge106. An electronic switch 102, in the form of a pair of antiparallel orbucking solid-state control rectifiers or thyristors 102a and 102b isconnected in series with the other section 111b of the primary winding 111. When the thyristors 102a and 102b are conductive and relay 103 isopen-circuited, the line current at 104a is applied across the fullprimary winding 1 11 to induce in the secondary winding 112 the normalwelding potential without step-up or stepdown. When, however, thethyristors 102a and 102b are nonconductive and relay 103 is closed, theline voltage is ap plied only across section 111a of the primary winding111 so that the voltage across the secondary winding 112 is stepped upsharply for purposes of ignition. The current detector 109 is connectedas represented at 110 to the gates of the controlled rectifiers 102a and1102b (via line 110a) and the relay 103 (via line 110k).

At the inception of the welding operation, thyristors 102a and arenonconductive while relay 103 is closed. Under these conditions, thevoltage applied across the electrode 113 and the workpiece 114 is of thehigher level (i.e., 200 to 800 volts) and ionization of the inert gas inthe gap 115 (see FIG. 3) is promoted. In fact, it is found that ignitionoccurs spontaneously almost instantly. As the gases ionized in the gap115 (the potential being above the breakdown potential of the gap)current flows in the form of an arc between the electrode 113 and theworkpiece 114 to produce a corresponding increase in the current flowthrough the welding circuit 107, thereby energizing the detector 109 andswitching, when the desired current level is reached, the relay 103 andboth thyristors 102a and 102b. The effective number of turns of theprimary winding is thereby increased with reduction of the potentialacross the workpiece and electrodes to the operating level, (up to 100volts) in short order.

In the system of FIG. 2, the full line voltage is applied at 2040 to theprimary circuit 204 and the untapped primary winding 211 while thesecondary winding 212 of the transformer 201 is tapped to provide thesections 2120 and 212b. The entire secondary winding is connected inseries with the relay 203 across the input terminals of the rectifierbridge 206 while a thyristor switch 202 (consisting of a pair ofantiparallel thyristors) is connected in shunt across the secondarywinding section 212b. The output terminals of the rectifier bridge 206are connected in series with the electrode, the workpiece, a choke 208,and a current detector 209 in the welding circuit 207 as previouslydescribed. The detector 209 operates the electronic switch 202 and theelectromagnetic switch 203 as represented by the lines 210, 2100 and2101:. At the inception of welding, the thyristor switch 202 isnonconductive while the electromagnetic switch 203 is closed circuited,thereby applying the full potential of the secondary winding 212, insteppedup configuration, across the bridge 206 and thereby energizingthe electrode and the workpiece with a DC voltage level above thebreakdown potential of the gap. As soon as the current begins to flow inthe circuit 207, the detector 209 renders the thyristors 202 conductivewhile open circuiting relay 203 to render only the section 212a of thesecondary effective,

thereby reducing the potential across the bridge 206 to the desiredoperating level.

FIG. 3 represents a system for the welding of stainless steel orzirconium alloy shells 314 of fuel elements for nuclear reactors. Theshell is here enclosed in a sealed chamber 319 filled with helium asrepresented by the tank 317, and receiving the welding head 313 which isspaced from the nuclear fuel element shell 314 by the gap 315. In thisembodiment, the transformer 301 has a tapped primary winding 311 whosesections 311a and 31112 are provided with a relay 303a, 303b and acontrolled rectifier switch 302, 302a, 302b, as described in connectionwith FIG. 1.

The secondary winding 312 of the transformer 301 is tapped and includesthe relay 303' whose coil 303b operates a switch 303a. A pair ofantiparallel controlled rectifiers 302a and 302b form an electronicswitch 302' performing a function similar to that described inconnection with FIG. 2.

In the welding circuit 307 connected across the output terminals of therectifier bridge 306, we have placed a choke 308 and a current-detectingdevice 309 whose taps are adjustable to pickoff a voltage dropcorresponding to the selected threshold current from the operation ofthe switching devices. The detecting resistance 3090 of the detectingcircuit 309 includes a signal amplifier 30% and a time-constant network3090 and is connected to the gates of the controlled rectifiers 302a,3021) and 302a, 302b.

During the inception of the welding arc, the switch 3030 of relay 303 isclosed while thyristors 302a and 302b are nonconductive and switch 303aof relay 303 is closed, whereas thyristors 302a and 302b arenonconductive. As a result, the primary voltage is applied only oversection 311a of the transformer primary while the secondary voltage istapped from the entire secondary winding 312, thereby stepping up thevoltage applied across the gap 315 and initiating the are as previouslydiscussed. When current flow commences across the gap 315, acorresponding potential is tapped at resistor 309a and is amplified at30% to reverse the switches 302, 302' and 303, 303'. After a period(determined by the time constant network 309c) of the order ofmilliseconds up to tenths of a second. The switches 303a and 3030' openwhile electronic switches 302 and 302 are rendered conductive, therebyswitching the voltage across the electrode and workpiece to the normaloperating level.

In FIG. 4, we show a modified system wherein independent sources areprovided for the working voltage and the superimposed potential. In thissystem, the working voltage is supplied by a transformer 401 whoseprimary winding is energized by the line current source 404 and whosesecondary winding operates a rectifier bridge 406 energizing theelectrode 413 and the workpiece 414 via a choke 415 and a currentdetector 409. The superimposed ignition potential is obtained from atransformer 401 of the step-up type whose secondary winding 412' isconnected across a rectifier bridge 406', the output terminals of whichare in parallel with those of the bridge 406. In the primary energizingcircuit of transformer 401 we have provided a thyristor switch 402 whichis triggered by the detectors 409; the latter also operates a relay 403whose switch contacts are in circuit with the primary winding oftransformer 401. At the inception of the welding operation, relay 403opens circuit transformer 401 and the higher DC voltage of transformer401' is applied across the gap. Upon detection of current flow in thesystem, circuit element 409 closes relay 403 and renders the electronicswitch 402 nonconductive.

The improvement described and illustrated is believed to admit of manymodifications within the ability of persons skilled in the art, all suchmodifications being considered within the spirit and scope of theinvention except as limited by the appended claims.

We claim:

1. An electrode, a workpiece spaced from the electrode to form an arctherebetween for direct current arc welding and for cutting of saidworkpiece, and a circuit for energizing said electrode and workpiece,the improvement wherein said circuit comprises:

a transformer having primary winding means connected to a source of linecurrent, and secondary winding means inductively coupled with saidprimary winding means;

rectifier means connected with said secondary winding means andenergizable thereby while having a direct current output connected tosaid workpiece and said electrode;

switch means including a pair of antiparallel connected thyristors incircuit with said transformer for temporarily increasing the effectivesecondary winding/primary winding turn ratio in a first condition ofsaid switch means and reducing said ratio in a second condition of saidswitch means;

current detector means in circuit with said rectifier means,

said electrode and said workpiece, and operatively connected with saidswitch means for switching same from said first condition to said secondcondition upon the generation of a discharge between said electrode andsaid workpiece when said switch means is in said first position; and

an electromechanical switch in circuit with said source and at leastpart of said primary winding means and closedcircuited upon switching ofsaid switch means from said first condition to said second condition.

2. The circuit defined in claim 1 wherein said primary winding meansincludes a first section and a second section, said source beingconnected across said first section, said thyristors being connectedbetween said source and said second section wherein said thyristorsapply said source across both said sections in a conductive state ofsaid thyristors but open circuit said second section in a nonconductivestate of said thyristors.

3. The circuit defined in claim l wherein said secondary winding meansincludes a pair of interconnected sections jointly connected across saidrectifier means, said thyristors being connected in shunt across one ofsaid sections for applying the other of said sections across therectifier means in a conductive condition of said thyristors butapplying both said sections across said rectifier means in anonconductive condition of said thyristors.

4. The circuit defined in claim 1, further comprising a choke connectedin circuit with said workpiece, said electrode and said rectifier means.

1. An electrode, a workpiece spaced from the electrode to form an arctherebetween for direct current arc welding and for cutting of saidworkpiece, and a circuit for energizing said electrode and workpiece,the improvement wherein said circuit comprises: a transformer havingprimary winding means connected to a source of line current, andsecondary winding means inductively coupled with said primary windingmeans; rectifier means connected with said secondary winding means andenergizable thereby while having a direct current output connected tosaid workpiece and said electrode; switch means including a pair ofantiparallel connected thyristors in circuit with said transformer fortemporarily increasing the effective secondary winding/primary windingturn ratio in a first condition of said switch means and reducing saidratio in a second condition of said switch means; current detector meansin circuit with said rectifier means, said electrode and said workpiece,and operatively connected with said switch means for switching same fromsaid first condition to said second condition upon the generation of adischarge between said electrode and said workpiece when said switchmeans is in said first position; and an electromechanical switch incircuit with said source and at least part of said primary winding meansand closed-circuited upon switching of said switch means from said firstcondition to said second condition.
 2. The circuit defined in claim 1wherein said primary winding means includes a firsT section and a secondsection, said source being connected across said first section, saidthyristors being connected between said source and said second sectionwherein said thyristors apply said source across both said sections in aconductive state of said thyristors but open circuit said second sectionin a nonconductive state of said thyristors.
 3. The circuit defined inclaim 1 wherein said secondary winding means includes a pair ofinterconnected sections jointly connected across said rectifier means,said thyristors being connected in shunt across one of said sections forapplying the other of said sections across the rectifier means in aconductive condition of said thyristors but applying both said sectionsacross said rectifier means in a nonconductive condition of saidthyristors.
 4. The circuit defined in claim 1, further comprising achoke connected in circuit with said workpiece, said electrode and saidrectifier means.